1. Field of the Invention
The present invention relates to a constant speed cruise control system for a vehicle with an automatic transmission (A/T).
2. Description of the Prior Art
A constant speed cruise control system of a duty ratio control type calculates the output duty ratio D by the formula ##EQU1## and adjusts the throttle valve opening degree accordingly. The car speed deviation .DELTA.V is a difference between the target car speed VM and traveling car speed V.sub.n (or spark advance car speed VS), and the set duty ratio SD corresponds to the target car speed VM.
The purpose of the constant speed control is to render the car speed deviation .DELTA.V equal to zero while adjusting the output duty ratio D, but when the set duty ratio SD is fixed, it is hard to establish .DELTA.V=0. Accordingly, a method of achieving .DELTA.V=0 by integrating and correcting the value of this SD has been proposed. In this integrating correction method, the set duty ratio SD changes, following up the output duty ratio D.
Incidentally, in the recent automatic transmission (A/T) cars, a four speed A/T with an overdrive function, (O/D) (first speed, second speed, third speed, and fourth speed=O/D) is in the main stream, and at car speeds of about 50 km/h or higher, the vehicle usually runs in O/D. But in the case of cruise control, traveling in the O/D state, constant speed traveling with O/D is difficult when climbing a steep slope, and the car speed is lowered by the lack of power needed for driving. In addition, when the carrying capacity of the car is increased, it is the same as the abovementioned case. To avoid this, therefore, an O/D cut-off signal is delivered from an ECU (Electronic Control Unit) for a cruise controller for an automatic speed change to cut off the O/D, and by raising the driving force, the reduction of the car speed is decreased.
For example, as shown in FIG. 1 (A), when the car speed is lowered by A (e.g. 4 km/h) from the target car speed VM, the O/D cut-off signal is turned on. This O/D cut-off signal is sent into a controller for an automatic speed change, and the selection of the speed change stage is defined. That is, the judgement of this controller for an automatic speed change involves a function of prohibiting the selection of O/D even if the speed in its range and causes a shifting down to the third speed. This function is not required when the car speed recovers up to VM-B (e.g. B=2 km/h), but the O/D cut-off signal is not turned off until the O/D reset timer expires (T.sub.OD, e.g. 14 sec). This is to prevent the frequent O/D cut-off and reset when the car speed has recovered quickly.
Meanwhile, the controller for an automatic speed change at the transmission side possesses a specific speed change pattern, and the gear is determined by the relationship between throttle opening degree and car speed. FIG. 2 is a speed change pattern between the third speed and fourth speed (O/D), in which the broken line indicates a region line rising shift-down and the solid line denotes a region line rising shift-up. When the control system is independent as shown here, if the ECU (electronic control Unit) for cruise control continues to open the throttle valve due to a speed reduction when climbing up a slope, the O/D may be cut off at the transmission side before the car speed is lowered to the O/D cut-off car speed (VM-A). Such a case is shown by VM-C in FIG. 1 (B). When the car speed rises as a result, the ECU for cruise control closes the throttle valve, and the transmission returns to O/D according to the speed change pattern. At this time, the car is in a state in which its driving power is deficient, and thus the car speed is lowered. As a result, the throttle valve is opened by a control signal output from the ECU for cruise control and the O/D is cut off at the transmission side. Hereinafter such an operation is repeated, and an O/D cut-off and reset are repeated in short time periods (that is, O/D hunting occurs).
Furthermore, if third-speed traveling results due to such an O/D cut-off, since the throttle valve opening degree is large due to the car speed being lowered at the O/D cutting-off, the driving force may be excessive unless the throttle valve opening degree at the O/D cutting-off is quickly returned, and it may lead to a car speed overshoot as shown in FIG. 3. The same holds true when shifting down to the third-speed independently at the side of the controller for an automatic speed change in the relationship between throttle valve opening degree and car speed (speed change pattern).